Ring source omnidirectional antenna



April 7, 1959 F. L. HENNESSEY RING SOURCE OMNIDIRECTIONAL ANTENNA FiledMarch 30, 1956 INVENTOR L. H EN N ESSEY FRANK ATTORNEYS United StatesPatent RING SOURCE OMNIDIRECTIONAL ANTENNA Frank L. Hennessey,Alexandria, Va., assignor to the United States of America as representedby the Secretary of the Navy Application March 30, 1956, Serial No.575,292

3 Claims. (Cl. 343-753) (Granted under Title 35, U. S. Code (1952), see.266) The invention described herein may be manufactured and used by theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor This inventionrelates in general to antennas and more particularly to broad band highgain antennas for use in radar and radio beacon systems operating atmicrowave frequencies.

.There has in the past been a concerted attempt to produce a radiationpattern having omnidirectional azimuthal characteristics yet of narrowbeam width in the vertical plane. Such a pattern finds utility forexample in radio beacon systems where it is desirable to transmit a maximum of energy in a substantially horizontal direction proximate to theground and in all azimuthal directions. The larger the verticalcomponent of such energy propagation the more reduced is the desirablehorizontal component. Stated differently, to achieve high energypropagation in let us say ahorizontal direction it is fundamental thatthe propagation in the vertical direction be limited; and, aspropagation in the vertical becomes more and more limited propagation inthe horizontal is increased.

While it has been suggested (see for example the Chu Patent No.2,486,589 and other patents cited therein) that a pattern of narrow beamwidth in the vertical plane might be achieved by devising an antennawherein the energy would emanate from a parabolic surface of revolution,such systems have heretofore, for one reason or another, not beenentirely satisfactory. One reason for the unsatisfactory operation ofthe prior art devices, as typified by Chu, lies in the inability toprovide a well defined source and only when the source is well definedand approaches a point source will the parabola provide a linear phasefront in the vertical plane.

A second problem existing in microwave frequency work involves theprovision of an antenna having an omnidirectional radiation pattern andat the same time a relatively small change of impedance with frequencychange. The present invention has been found to more adequately meetthese two requirements than other known antenna types.

Accordingly, it is one object of this invention to provide an antennasuitable for operation over an extremely wide band of frequencies.

It is another object of this invention to provide a broad band antennahaving omnidirectional characteristics.

Another object of this invention is to provide an antenna with high gainin the horizontal plane.

Another object of this invention is to provide an antenna having aradiation pattern of limited vertical height and omnidirectionalcharacteristics.

Another object of the present invention is to provide an antenna havingan extremely narrow beam width in the vertical plane.

A further object of this invention is to provide an antenna having aradiation pattern comprising parallel beams of rays in the verticalplane and of omnidirectional characteristics.

A still further object of this invention is the provision of an antennastructure possessing great rigidity and ruggedness and yet economical tomanufacture.

Other objects and features of the present invention will become apparentupon consideration of the following detailed description when taken inconnection with the accompanying drawings wherein:

Fig. 1 is a cross-sectional view of an antenna structure according toone embodiment of the invention;

Fig. 2 is a view of the antenna structure as seen when looking towardthe feed or ring shaped aperture of the same; i

Fig. 3 is a cross-sectional view of the antenna structure showing theelements slightly reoriented;

Fig. 4 shows the antenna structure coupled to a circular waveguide; and

Fig. 5 shows still another modification of the instant invention.

The present invention constitutes a broad band antenna for use in radiobeacon systems operating at ultra-high frequencies. As stated previouslyfor such operation it is desirable to produce an energy radiationpattern having omnidirectional azimuthal characteristics yet ofextremely narrow beam width in the vertical plane. As the beam width inthe vertical plane is decreased energy propagation in the horizontalplane is proportionally increased. To achieve this desired radiationpattern the present invention provides an antenna structure wherein theenergy to be propagated converges from a ring aperture toward a surfaceof revolution. The surface of revolution serves to redirect the energyand is of such a configuration that the energy appears to emanatetherefrom as substantially parallel rays in the vertical plane. When therays leave the antenna at right angles to the axis thereof maximumradiation in the horizontal plane is assured.

One modification of the invention comprises a parabolic surface ofrevolution for reflecting the energy leaving the ring aperture. Anothermodification contemplates a surface of revolution in the form of a lensto refract the rays of energy leaving the ring aperture.

Referring now to the drawings and more particularly to Fig. 1, there isshown a pair of spaced, circular, parallel plates 1 and 2. Plate l isprovided with an inturned portion 4 near its center and a peripheral,laterally extending, flange portion 5. The inwardly turned portion 4abuts and is secured to the inner conductor of the coaxial cable 3 andserves to introduce the energy from the coaxial cable into the mediumbetwen the parallel plates without the energy encountering any abruptchange of impedance. While the inner conductor of the coaxial cable 3has been shown as hollow in cross-section it is understood that it couldalso be solid, as is probably more common. Plate 2 is likewise inturnedas at 6 and abuts and is secured to the outer conductor of the coaxialcable 3. Plate 2 is also provided with a peripheral, laterallyextending, flange portion 7. The flanged portions 5 and 7 definetherebetween a ring shaped aperture 8 which serves to direct the energyintroduced into the medium between the parallel plates toward thesurface of revolution 9.

The surface 9 can be described geometrically as; that surface generatedby rotating a parabola y =4px around the vertical axis. The energyleaving the ring shaped aperture 8 converges upon the parabolic surfaceof revolution 9 and is reflected thereby. Paraboloid 9 may be of hollowconstruction or may be a shaped block of' with a slightly flared lip forinsuring proper directivity- ;of the energy converging onto surface 9.For optimumperformance the energy should illuminate substantially all ofthe curved surface of the parabola from its vertex to its terminatedend. Further, for optimum performance the aperture 8 must not be toosmall nor too large. Too small an aperture will of course provideinsufficient directivity to the energy and the same will spew all over,much even missing the paraboloid. Too large an aperture will result inonly a segment of surface 9 being utilized. The exact width of aperture8, will of course be dependent upon the height of the surface 9 which isto be illuminated.

Thus, the coaxial cable 3 introduces the microwave energy axially intothe medium between the parallel plates. and it travels to aperture 8where it is directed toward surface 9. This surface, in the form of aparabolic surface of revolution, reflects the energy and the same ispropagated out into space in all directions at substantially rightangles to the axis of the antenna. The energy thus appears in the formof parallel rays of beams in the vertical or axial direction and ofomnidirectional azimuthal characteristics. Since the energy leaves theantenna as parallel rays in the vertical direction, a radiation patternof limited vertical height is achieved. The height of the surface ofrevolution 9 will of course determine the vertical beam width of theradiation pattern. For optimum performance this paraboloid should be ofa height of at least eight wavelengths, at the lowest frequency in theoperating range of the antenna. The spacing between the parallel plates,while not critical, should be something less than a half wavelength atthe highest operating frequency. The radius of the parallel plates isnot at all critical.

A circular insulating ring 10 is placed between the plates 1 and 2 andserves primarily as a support for plate 1. Insulator 11 is in the formof a surface of revolution and functions both as a support and as ameans of insuring the symmetrical positioning of the inner conductor ofcoaxial cable 3 with respect to parallel plates 1 and 2. Any asymmetryof the cable with respect to these plates would distort theomnidirectional characteristics of the antenna pattern. As shown in thedrawing, the side edges of the insulators 10 and 11 are tapered toreduce mismatch. The element 12 is merely the common type insulator usedin coaxial cables.

While it has been found that a parabolic surface of revolution is themost desirable for the intended purpose, it should be understood thatother surfaces of revolution could be used especially when radiationpatterns of different vertical configuration are desired. For example,should it be desired to provide an omnidirectional pattern having alop-sided vertical beam, for example one with an increased upwarddirectivity, the surface 9 could readily be modified to provide thesame. Again, should it be desired to have maximum radiation in adirection above or below the horizon at some prescribed angle it wouldbe necessary only to use as the generatrix for the reflector a sectionof a parabola of which the line between its vertex and focus makes thedesired angle with the horizontal. The exact surface necessary toprovide a prescribed vertical beam pattern can be arrived at throughmathematical or empirical means.

Fig. 2 shows the antenna structure when looking toward the open aperture8. This figure has been used to merely point up the circularconfiguration of the elements.

Fig. 3 shows an antenna structure similar in most respects to that ofFig. 1 except for the fact that the elements thereof are slightlyrearranged. In this embodiment the plates 1' and 2 are arranged to havethe ring shaped aperture 8' direct the energy upwardly toward theparabolic surface of revolution 9. The paraboloid is inverted so thatthe energy striking it is reflected outwardly at right angles to theaxis of the antenna. In the sense that the parabolic reflecting surfaceredirects the rays of energy to render the same parallel in the verticalplane, and at right angles to the axis of the antenna, it functions as acollimator.

In Fig. 3, the coaxial cable 3 approaches the plates 1' and 2 from adifferent direction. For this arrangement it is necessary to couple theouter conductor of cable 3' to plate 1 and the inner conductor to plate2. Except for these modifications, this antenna structure functions inthe same manner as the antenna structure of Fig. 1.

Turning to Fig. 4, an antenna structure, substantially the same as thatof Fig. 1, is shown coupled to a circular Waveguide 16. The microwaveenergy is transmitted in the circular waveguide in the TM mode and isintroduced axially into the medium between the parallel plates. Thepointed nub 17 assists in the introduction of the energy into the mediumwithout it encountering any abrupt change of impedance. In all otherrespects this antenna is the same in structure and operation as theantenna of Fig. l.

The antenna of Fig. 5 is different from those previously described inthat the energy is refracted or focused by a lens. The microwave energybeing transmitted by coaxial cable 3" is axially introduced into themedium between parallel plates 1" and 2" and thence travels to ringshaped aperture 8" where it is directed toward the lens 9". Lens 9 is inthe form of a surface of revolution aligned with and symmetrical withrespect to the axis of the parallel plates; and, is preferably made of aplastic material such as polystyrene or the like.

The lens 9 functions as a collimator in that it redirects the rays ofenergy leaving aperture 8" and renders them parallel in the verticalplane. The configuration of the lens surface can he arrived atempirically and is such that all the rays leaving a point source,located at aperture 8", have the same electrical path length. That is,the electrical path length from a to b will be equal to that from a toc, a to d, and a to e. Thus, the energy leaving the antenna in any onedirection possesses a linear phase front in the vertical. It should beclear at this point that the energy converging on the surface ofrevolution from any one azimuthal direction will, for all practicalpurposes, appear to be coming from a point source.

As has been stated previously, the structure of the instant inventionpossesses very broad banded characteristics, considering of course thefact that its radiation pattern is of very high directivity in thevertical plane. Typically, the voltage standing wave ratio, for one ofthe models tested, was found to be less than 1.5 over a band from8200-9600 megacycles.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A broad band antenna system comprising: a microwave transmissionline, a pair of spaced plates centered on a common axis with the majorplanar surfaces of said plates in parallel planes, said transmissionline being coupled to said plates to introduce microwave energy into thespace between said plates, at least one of said plates having aperipheral flange portion for directing said microwave energy from saidspace in rays toward a portion of said common axis, and a lens meanspervious to microwaves centered on said portion of said common axis forretracting said microwave energy so that said rays in any planecontaining said common axis are substantially parallel.

2. A broad band antenna system comprising, a microwave transmissionline, a pair of spaced plates, the major planar surfaces of said platesbeing parallel and centered on a common axis, said plates being coupledto said transmission line to introduce microwave energy into the spacetherebetween, at least one of said plates having a peripheral flangeportion for directing said energy from said space toward a portion ofsaid common axis, and a lens of circular cross-section and pervious tomicrowave energy centered on said portion of said common axis with theaxis of revolution of said lens aligned with 5 said common axis, wherebysaid lens rcfracts said microwave energy in a predetermined patternaround said common axis.

3. A broad band antenna system for microwave energy comprising: asection of coaxial transmission line, a first 10 nection to said firstplate, said first plate having a central aperture through which saidinner conductor passes, said inner conductor being terminated byconnection to the center portion of said second plate, said lensconsisting of a dielectric solid of revolution pervious to microwaveenergy connected to said second plate, and flange means connected to atleast one of the peripheries of said first and second plates fordirecting microwave energy from the space between said plates to thesurface of said lens, whereby said lens refracts said energy in apredetermined pattern around the said common axis.

References Cited in the file of this patent UNITED STATES PATENTS TinusApr. 17, 1951

